1. Field of the Invention
The present invention relates to a calibration method for mechanical parameters of an industrial multi-jointed robot and a measuring system automatically carrying out the calibration method. In particular, the invention relates to a system and a method for automatically measuring the positional relationship between a front end of a robot and the displacement of a controllable driving axis of the robot, by means of a light-receiving device capable of detecting a target or an object to be measured, and for calculating the dimension error of a mechanical unit of the robot and an error capable of being formulated, for example, elastic deformation thereof.
2. Description of the Related Art
In order to position a front end of a robot at a desired position, it is necessary to determine various parameters which specify the relationship between the position and the orientation of a front end or a hand reference point of the robot (for example, the position and the orientation of a face plate coordinate) and the angular displacement of each robot axis. However, these parameters inevitably include errors which occur during machining or assembling the robot. Further, it is difficult to directly measure these parameters of the assembled robot. Accordingly, in an industrial use, a calibration method, for measuring the relationship between the input displacement and the output displacement of the robot, and estimating the optimum value based on the measurement result so as to correct the parameters, is often utilized.
Various devices and methods regarding the calibration are known. For example, Japanese Unexamined Patent Publication No. 2001-38662 discloses a calibration method for measuring the position of a front end of a robot by fixing a three-dimensional measuring device to a stationary coordinate system. Japanese Unexamined Patent Publication No. 61-133409 discloses an automatic correction method for executing teaching in which a front end of a tool of a robot is directed to one three-dimensional arbitrary point. Also, Japanese Unexamined Patent Publication No. 4-259007 discloses a correction method for measuring the distance between a front end of a robot and one three-dimensional arbitrary point by means of a three-dimensional measuring device arranged on the front end of the robot. Further, Japanese Unexamined Patent Publication No. 6-241729 discloses a calibration method for executing teaching in which a camera for a robot may capture a target at the center of the field of vision of the camera, relational to a plurality of positions or orientations of the camera. Further, Japanese Unexamined Patent Publication No. 5-8185 discloses an automatic measurement method for calculating errors of all link elements of a robot mechanical unit, based on the difference between an imaging result and a theoretical figure, the imaging result being obtained by an imaging device arranged on a front end of the robot such that the imaging device at various postures may image a fixed point.
In order to obtain a high degree of accuracy in the conventional calibration, a highly accurate measurement is required. On the other hand, workability of calibration is also required. The measuring device disclosed in Japanese Unexamined Patent Publication No. 2001-38662 is generally large, therefore, the device is difficult to be used in a narrow working area. In the method of Japanese Unexamined Patent Publication No. 61-133409, since the teaching is manually carried out without using a measuring device, a teaching error may be generated.
On the other hand, the device disclosed in Japanese Unexamined Patent Publication No. 4-259007 or Japanese Unexamined Patent Publication No. 6-241729 is relatively small. However, the teaching of the orientation of the robot during measurement is carried out manually. Also, in Japanese Unexamined Patent Publication No. 6-241729, since only two-dimensional view lines of the camera are aligned, data obtained by one measurement operation is fewer than that obtained by using a three-dimensional measuring device. Therefore, more postures are required for the camera when imaging. In Japanese Unexamined Patent Publication No. 61-133409 and Japanese Unexamined Patent Publication No. 6-241729, since an operator is required to position the camera for measurement, the measurement result depends on the skill of the operator. In addition, since it is necessary to set a large number of teaching points in order to reduce the adverse effect of any teaching errors, the workload on the operator may be increased.
In Japanese Unexamined Patent Publication No. 5-8185, the accuracy of measurement depends on an error of the imaging device, such as the distortion of a lens. Further, the position of the fixed point to be measured must be previously known in order to carry out the automatic measurement. However, a jig or the like must be made and attached to a suitable place.